1. Field of the Invention
The present invention relates to a magnet unit and a magnetron sputtering apparatus and, more particularly, to an improvement in the structure of a magnet unit arranged on the rear surface side of a cathode electrode supporting a sputtering target and a magnetron sputtering apparatus having the magnet unit.
2. Description of the Related Art
A sputtering process used in deposition in the semiconductor industry can deposit a film of any material including, for example, a refractory material such as platinum and tungsten, or an insulating material such as SiO2. In addition, it is easy to change the energy of sputtering particles, and it is also possible to control, for example, the crystalline characteristics, magnetic characteristics, insulation characteristics, and stress of a film.
A sputtering cathode used in the sputtering process adopts the following principles. A cathode magnet is arranged in the atmosphere behind a target arranged in a vacuum, with a partition (for example, a back plate) between them. Magnetic lines of force formed by the cathode magnet generate a magnetic tunnel that forms an endless annular trajectory on the flat surface of the target (note that a set of points at which a component, perpendicular to the flat surface of the target, of the magnetic tunnel is zero will be referred to as a “magnetic track”). In this state, supplying a power to the target generates an electric field in the direction of the normal to the surface of the target. Electrons are confined in a region created when the magnetic field and the electric field intersect at right angles. When the confined electrons collide against gas atoms many times, the gas atoms turn into ions. The electric field on the front surface of the target accelerates the ions, thereby causing sputtering.
Since the ions sputter the atoms on the target surface, the target surface erodes (to be referred to as “erosion”) over the use time. When the depth of erosion gets close to the thickness of the target, the target needs to be exchanged with a new one. If erosion concentrates on a certain position and the erosion speed becomes high, the target exchange frequency also becomes high, thereby decreasing the availability of a sputtering apparatus. On the other hand, if the use efficiency of the target is high and the erosion speed is low, the target exchange frequency is low, thereby increasing the availability of a sputtering apparatus.
The erosion speed changes depending on factors such as an electric field strength and a magnetic flux density generated on the front surface of the target, a sputtering gas pressure, and a magnetic track shape. The erosion often selectively proceeds in a partial region (partial diameter) of the target surface, thereby raising the erosion speed.
To reduce a concentration of erosion, a magnetic track shape, that is, a cathode magnet shape (magnetic circuit) has been mainly improved, for which many techniques have been proposed. It is, however, difficult for a linear plasma generated on a magnetic track to erode the whole wide flat surface of the target evenly. Therefore, a method of eroding the whole flat surface of the target by rotating or swinging (performing reciprocation for) the magnetic track (cathode magnet) is used.
Japanese Patent Laid-Open No. 63-317671 proposes a cathode magnet in which, as shown in FIG. 25, a second magnetic apparatus 133 is arranged between the N and S poles of a first magnetic apparatus 131 provided on the rear surface of a target, and the N and S poles of the second magnetic apparatus 133 are alternately arranged, in the extending direction of the N and S poles of the first magnetic apparatus 131, to be spaced apart from each other and to face the surface side of the target. Note that reference numeral 151 in FIG. 25 denotes the centrode of an electron e.
In Japanese Patent Laid-Open No. 2001-348663, as shown in FIG. 26, there are provided a backing plate which is connected with a power supply and has a function as a cathode electrode, a target attached on the surface of the backing plate, and a magnetic circuit arranged on the rear surface of the backing plate to face the target. The magnetic circuit is arranged so that an erosion region A appearing on the surface of a target 281 is made into meandering closed curves.
In Japanese Patent No. 4175242, as shown in FIG. 27, there is proposed a magnetron sputtering apparatus in which a target 392 is arranged in a vacuum chamber 391. The apparatus includes, on the rear surface of the target, an inner magnet 394, an outer magnet 395 which has a magnetization direction opposite to that of the inner magnet 394 and surrounds the inner magnet 394, and a yoke 396 arranged to face the target 392 and to sandwich the inner and outer magnets therebetween, and further includes a magnetic circuit for generating arcuate magnetic lines of force 397 on the surface of the target 392. Moreover, a horizontal magnet 311 which has a magnetization component that repels those of the outer magnet 395 and inner magnet 394 is inserted between the outer magnet 395 and inner magnet 394 to be parallel to the surface of the target 392.
In the methods proposed in Japanese Patent Laid-Open Nos. 63-317671 and 2001-348663, the magnetic circuit of the cathode magnet arranged behind the target is formed into a wavy shape, thereby improving the use efficiency of the target while preventing sputtering particles from adhering again on the target and also preventing a concentration of erosion.
If, however, the second magnetic apparatus 133 is arranged within the first magnetic apparatus 131 as in Japanese Patent Laid-Open No. 63-317671 (FIG. 25), the S- and N poles are close to each other. That is, magnetic lines of force close directly above the cathode magnet, and they do not appear on the surface of the target. As a result, a magnetic field does not appear on the target. If the S- and N poles of the second magnetic apparatus 133 are moved away from the S- and N poles of the first magnetic apparatus 131, magnetic lines of force never close directly above the cathode magnet, and a magnetic field appears on the target. The method, however, increases the size of the first magnetic apparatus 131, and then the cathode becomes large.
On the other hand, in the structure in which an outer magnet 221 surrounds a meandering inner magnet 220 with some distance as in Japanese Patent Laid-Open No. 2001-348663 (FIG. 26), the width of the outer magnet 221 which is interlocked with the curved portion of the inner magnet 220 is limited. That is, as the width of the curved portion of the inner magnet 220 becomes large, the width of the outer magnet 221 becomes small. Consequently, the magnetic field of the curved portion weakens, and the meander width of the magnetic track becomes small. Furthermore, if a region A is filled with magnets to reserve the meander width, it is possible to widen the meander width but the magnetic field strength becomes locally high, and erosion locally proceeds quickly, which makes the meandering form meaningless.
In Japanese Patent No. 4175242 (FIG. 27), since the horizontal magnet 311 is arranged between the outer magnet 395 and the inner magnet 394 to be parallel to the surface of the target 392, the magnetic flux densities of the outer magnet 395 and inner magnet 394 are equal to each other. Therefore, it is impossible to form a wavy magnetic circuit.